The present invention relates to bumper systems using energy absorbers, and more particularly to automotive polymeric energy absorbers such as for a vehicle front or rear bumper. However, a scope of the present invention is not believed to be limited to only bumper energy absorbers and/or bumper systems.
Many automotive bumper systems include metal reinforcement beams and polymeric energy absorbers tuned for very specific energy absorption profiles, including “softer” initial impact strokes for pedestrian safety and “harder” deep-stroke impacts such as for vehicle-to-stationary-object impacts. Consistency and predictability of energy absorption over the bumper system's stroke during an impact is very important. Many energy absorbers have multi-sided crush boxes for consistent and predictable energy absorption. For example, see the energy absorbers shown in Weissenborn U.S. Pat. No. 6,575,510 and Evans U.S. Pat. No. 6,609,740.
However, complexity of the energy absorber adversely affects the lead times required by mold shops for providing the molds, and further adversely affects moldability of the parts, such as by adding to die cost and mold cycle times and maintenance. Complexity can come from many things, including the shape, size, and/or number of crush boxes required. This results in conflicting design requirements. On the one hand, energy absorbers can be designed to have smaller crush boxes, but the molds (and the molding process) become more complex due to an increased number of crush boxes required to cover the front surface area of the bumper reinforcement. Further, the energy absorbers become much heavier due to additional material in the walls of the “additional” crush boxes, and mold cycle times increase due to the additional material. Also, molds can become more complex due to undulations and other structure placed in the walls. Energy absorbers are easier to mold (and lighter in weight) when they have larger crush boxes, but large unsupported areas in the walls of the crush boxes tend to be unstable, leading to premature collapse during impact (i.e., inconsistency) and/or poor energy absorption. Efforts to stabilize the walls usually add to part weight (such as by adding to material thickness of the walls) and/or to part complexity (such as by adding attachments to anchor the walls in predetermined positions).
Another consideration is the lead times in bumper development programs. The vehicle industry is under immense pressure to reduce lead times for designing and bringing a product to market. Yet, tooling must often be started early in the program. The reinforcement beam and energy absorber are then “tuned” in order to obtain optimal energy absorption profiles (i.e., force versus displacement). “Tuning” of bumper systems can be difficult and time consuming in and of itself, particularly when the molds must be sent offsite in order to machine in thicker walls or new wall shapes.